Is there an energy density limit in GR? I am speaking about GR with classical fields and energy. One question, spread over three increasingly strict situations:
Is there an energy density limit in GR? (literally, can the energy density have an arbitrarily large value at some point in space at some point in time)
Is there an energy density limit beyond which a blackhole will always form?
Let's choose a small volume, for here I'll just choose the Planck volume.  Is there an average energy density limit over this volume beyond which a blackhole will always form?
Clarification:
In light of http://en.wikipedia.org/wiki/Mass_in_general_relativity , can those that are answering that the energy density is limited and referring to a mass $M$ in some equations please specifically state how you are defining the $M$ in terms of the energy density, or defining $M$ in terms of $T^{\mu\nu}$ the stress-energy tensor.  Does your $M$ depend on coordinate system choice?
Also, reading some comments, it sounds like there is confusion on what energy density means.  Based on wikipedia http://en.wikipedia.org/wiki/File:StressEnergyTensor.svg , it sounds like we can consider energy density = $T^{00}$ of the stress-energy tensor.  If you feel this is not correct terminology, please explain and I'll edit the question if necessary.
 A: The answer is NO.  There is no energy density limit (for all three questions).
The easiest way to see this is that the energy density is just the $T^{00}$ component of the stress energy tensor.  The solution in GR depends on the full stress energy tensor, so it is not enough to just talk about the energy density.  Furthermore, because the energy density is just a component of a tensor, it is a coordinate system dependent quantity.  So starting from a solution that doesn't become a blackhole, and has some energy somewhere, we can always choose the coordinate system to make the energy density arbitrarily large.
More clearly stated: Local Lorentz symmetry alone is enough to show that the energy density is not limited in GR.  And furthermore since there exist non-zero energy solutions that don't become blackholes, this also answers your second question.
To make the answer to the third question more clear, let's discuss an exact solution.  Consider the Robertson-Walker solution with a perfect fluid.  Here's an example stress energy tensor for a perfect fluid in the comoving frame:
$T^{ab} =\left( \begin{matrix} \rho & 0 & 0 & 0 \\ 0 & p & 0 & 0 \\ 0 & 0 & p & 0 \\               0 & 0 & 0 & p \end{matrix} \right)$
Now if we change to a different coordinate system, using the coordinate transformation:
$\Lambda^{\mu}{}_{\nu} =\left( \begin{matrix} \gamma &-\beta \gamma & 0 & 0 \\ -\beta \gamma&\gamma&0&0\\ 0&0&1&0\\ 0&0&0&1\\ \end{matrix} \right)$
We see the energy density will transform as:
$\rho' = \gamma^2 \rho + p \beta^2 \gamma^2 = \gamma^2 (\rho + p \beta^2)$
So not only can the energy density be arbitrarily large, but even over a finite volume.
A: The Schwarzschild radius is $R~=~2GM/c^2$, where if you pack a mass $M$ into a volume with a radius $R$ you get a black hole.  The term mass-energy limit is not standard language usage, but if you push sufficient mass into a volume it will becomes a black hole and causally sealed off from the outside world.
A: There is an energy density limit in physics (imagine a Hubble volume full of photons with a wavelength reduced to a plank length packed 1 per planck volume, collapsing into a singularity). However general relativity has no fundamental quantities besides c and thus no such constraint.
